The present invention relates to steel sheet, coil or plate (hereunder collectively referred to as steel sheet) with a multilayer electroplating that has high affinity for chemical conversion treatment (e.g. phosphating, hereinafter sometimes referred to as "phosphating") and can be coated with a paint coating having improved wet adhesion.
Steel sheets with platings of Zn or Zn-based alloys such as Zn--Ni and Zn--Fe systems (hereunder collectively referred to as steel sheets with Zn-base coating) are extensively used as corrosion-resistant materials in automotive parts and electrical appliances. Essentially, the mechanism of corrosion protection in these sheets consists of the electrochemical sacrificial protection of the steel substrate by the plating, or the formation of a protective film on the surface of the plating under corrosive environment.
In consideration of driving on salt-spread roads in cold districts, recently built automotive bodies are usually painted by cationic electrodeposition. However, steel sheets with Zn-base coating do not have good adhesion to paint coatings formed by cationic electrodeposition, and it is very difficult to provide by commerical application techniques a coating that has sufficient wet adhesion for use in automotive bodies.
Another problem with the steel sheet with Zn-base coating is that surface flaws such as tiny craters are easily formed in the paint coat formed by cationic electrodeposition. It is thought that these craters are formed by hydrogen gas evolved upon current impression during electrodeposition. They are very detrimental to the commercial value of the final product since they make it unsightly and reduce its corrosion resisting properties.
Under these circumstances, steel sheets with various types of duplex plating comprising two layers of plating have recently been proposed. Of these proposals, the one having the greatest potential for commercialization is a steel sheet with a continuous Fe or Fe--Zn base (Zn.ltoreq.40 wt %) outer coating and an inner Zn-base protective plating, as described in Japanese Patent Application Laid Open No. 133488/81 and No. 142885/81. This steel sheet has much better adhesion to electrodeposited paint coating than products having only a Zn-base plating, and it is quite effective in suppressing the formation of craters during paint application by electrodeposition. However, this dual plating system has its own problems.
The theory behind this dual plating is that the inner layer provides corrosion resistance whereas the outer (surface) layer provides good adhesion to the paint film. The following points must be taken into consideration when determing the quality of the Fe or Fe--Zn base alloy coating forming the surface layer. (1) The most important factor for providing a paint film having good wet adhesion is the affinity of the steel substrate for phosphating. The greater the amount of phosphophyllite (Zn.sub.2 Fe(PO.sub.4).sub.2.4H.sub.2 O) that is formed in the phosphate film, the more wet-adhesive to the substrate the paint film is, and the opposite phenomenon occurs if more hopeit (Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O) is formed. The Fe or Fe--Zn alloy surface layer contributes to the production of a phosphophyllite crystal in which the constituting Fe is supplied from the surface layer, so the minimum amount of the deposition of this surface layer should be sufficient to supply Fe necessary for producing a dense phosphophyllite crystal. (2) Another factor for determining the lower limit of the deposition of the surface layer is the need to inhibit the formation of craters during cationic electrodeposition. Generally, the phosphate film does not have a 100% coverage (under the most ideal conditions, about 0.1 to 1% of the substrate area is still exposed). The Fe-rich film just beneath the phosphate coating is capable of inhibiting the formation of craters in subsequent cationic electrodeposition. As described in (1) above, the surface layer is dissolved during the phosphating operation, but to suppress the formation of craters, at least part of the surface layer must remain until the end of electrocoating. Therefore, the amount of the deposition of the surface layer must be such that not all of the layer is consumed during the phosphating. (3) On the other hand, the surface layer has a high Fe content, so if it is deposited in an excessive amount, a scratch, no matter how small it may be, will be a rust developing site. What is more, this surface layer has inherently high internal stress and does not have a very good adhesion to the inner layer. If the deposition thickness of this surface layer is increased, press working entailing increased strain causes heavy "powdering". To prevent this, the deposition of the surface layer must be held to a minimum. (4) In addition, the essential purpose of the surface layer is not to provide corrosion protection, so economy dictates that it be as thin as possible. In electroplating, the thicker the plating, the higher the variable cost, and the greater the size of the plating equipment, the higher the initial cost.
In view of the discussion above, the Fe-- or Fe--Zn alloy surface plating in the duplex coating system must be as thin as possible, provided that it is thicker than the critical value necessary for providing good affinity for phosphating and preventing the formation of craters during cationic electroposition, the preferred thickness being not more than about 10g/m.sup.2.
As stated in Japanese Patent Application Laid Open No. 133488/81 and No. 142885/81, the surface layer of the duplex coating must be "continuous" to cover the whole area of the inner Zn-base corrosion-resistant layer (see accompanying FIG. 1(a) wherein the inner and surface layers are indicated by numerals 1 and 2, respectively). If the surface layer 2 is discontinuous and does not cover the inner layer 1 entirely as shown in FIG. 1(b) and if the electrochemical potential of the surface layer is cathodic to the inner layer, the inner layer is preferentially dissolved during phosphating, and not enough Fe is supplied from the surface layer to form a dense phosphophyllite crystal. The inner Zn-base (Zn, Zn--Fe or Zn--Ni) plating that must provide corrosion protection is usually anodic to the Fe-- or Fe--Zn alloy surface plating (of high Fe content).
Because of the mechanism of deposition, an electroplated coating may often be discontinuous. In the electroplating of a metal, a multitude of active sites dispersed on the substrate serve as nuclei for starting the deposition of the metal, and the deposition of metal spreads not only in the direction of thickness but also in every direction in the plane until a continuous film that covers the whole area of the substrate is formed. The sequence of this formation is illustrated in FIGS. 2(a), 2(b) and 2(c), wherein symbol A represents the plating film. Therefore, in some cases, the plating operation comes to an end in stage (a) before the deposited film A has formed a continuous layer, and the resulting surface layer is discontinuous like surface layer 2 in FIG. 1(b) wherein micro-pores 3 are randomly distributed throughout the coating. For the mechanism of the formation of an electroplated film and its discontinuity as well as associated phenomenon, see the following references.
(i) J. A. Harrison & H. R. Thrisk, "Advances in Electrochemistry and Electrochemical Engineering", Vol. 8, page 97, Interscience Pub., John Wiley & Sons Inc. (outlining the mechanism of the formation of a plating by electrodeposition);
(ii) "Metallic Coatings for Corrosion Control", NewnesButterworths, 1977, particularly the articles entitled "Effects of discontinuities in coating" and "Anodic coatings", pp. 39-41 and FIG. 1.17;
(iii) "Properties of Electrodeposits--Their Measurement and Significance", The Electrochemical Society, Inc., 1975, particularly the paper entitled "Porosity and Porosity Tests", p. 122 (the second and third references describe the discontinuity of electroplated deposits and the galvanic corrosion resulting from such discontinuity).
In short, the Fe-- or Fe--Zn surface layer of the duplex plating should be not only as thin as possible (usually not thicker than 10g/m.sup.2) but also continuous, rather than discontinuous as shown in FIG. 1(b). However, as will be readily understood from the mechanism of its formation, the discontinuous layer is more often formed when the plating is thin than when it is thick. Therefore, the two requirements that the plating be continuous and not thicker than 10g/m.sup.2 are generally very difficult to meet, and the conventional technique of using a simple bath, made up of a sulfate salt or chloride comprising Fe.sup.2+ or Fe.sup.3+ and optionally Zn.sup.2+ supported by an inorganic salt (e.g. Na.sub.2 SO.sub.4, NaCl or Al.sub.2 (SO.sub.4).sub.3) is practically ineffective.
For the formation of micro-pores in thin layers of electroplating, see the following references.
(iv) "Effects of discontinuities in coating", p. 40 of Reference (ii), disclosing the inverse relation between the thickness of the electrodeposit and the number of micropores present;
(v) "Trend and Future of Surface Finishing Techniques in Steel Industry" in "The 52 and 53rd Nishiyama's Commemorative Lecture: Advancement of the technology for manufacturing surface-finished products and associated fileds", The Iron and Steel Institute of Japan, 1978 (showing the formation of a Sn-plating of a thickness of less than 5 g/m.sup.2 which is discontinuous and entails micro-pores);
(vi) Japanese Patent Publication No. 42774/73 (showing that an electroplated Zn coating has a number of pinholes or micro-pores when its thickness is less than 10 g/m.sup.2); and
(vii) "The sine-wave pulse plater" in "International Pulse Plating Symposium", American Electroplaters Society, Inc., 1979) showing the high porosity of an electroplated gold film thinner than 3 .mu.m).
A conventional method of reducing the porosity (micropores) of a thin plating film consists of adding to the plating bath a complexing agent, such as a chelating agent, a cyanide, an organic acid (e.g. citric acid or succinic acid), or an organic additive (e.g. glue, dextrin, tetrabutyl ammonium, bromide, or benzalacetone). However, this method has a problem that prevents its extensive use: the amount of the complexing agent in the bath varies with a continued fluctuation in operating conditions, whereas measurement of its concentration is quite difficult making quality control of the bath infeasible.
Even if the desired continuous thin surface plating is successfully produced by this method, an unavoidable problem arises when the steel sheet with this coating is put to service. The sheet is usually subjected to various forming operations such as press working by the user such as a car manufacturer, and then it is chemical-converted, e.g. phosphated and painted, but the deformation applied to the work in the car manufacturing process is usually relatively severe, hence discontinuities (micro-cracks) often develop in the surface of the Fe-- or Fe--Zn plating which inherently has high internal stress. In other words, it is highly likely that discontinuity (porosity) will be introduced into the plating film during the forming step even if it was suppressed during the platingstage. As already mentioned, a duplex coating having such a discontinuous surface plating layer cannot have improved affinity for phosphating. Specific data of the experimental work done to investigate the introduction of discontinuities into an electrodeposit by forming operation and the resultant effects are reported in the following reference:
(viii) "Steel Sheet with Zn--Fe/Zn--Ni Duplex Alloy Electroplating --Its Workability and Corrosion Resistance after Working" in "Metal Surface Finishing", Vol. 33, No. 10, pp. 505-508, 1982 (showing the presence of micro-cracks in the surface Fe--Zn layer of the Fe--Zn/Zn--Ni duplex electrodeposit on the steel sheet that has been subjected to forming operation; since the inner Zn--Ni layer is cathodic to the surface layer, the phosphophyllite content of the phosphate film is decreased by a degree that depends on the severity of the mechanical deformation).
As described in the foregoing, the steel sheet with Fe-- or Fe--Zn base/Zn--base duplex coating has many problems to be solved before it can be extensively used on a commercial scale. None of the existing plated steels are completely satisfactory as commerical products.